Synchronizing a receiver with a received signal can be achieved by estimating symbol timing, carrier frequency offset, carrier phase offset, and/or sampling rate offset. Synchronization can be performed when the receiver begins operating and can continue while the receiver is operating. This continuing synchronization is sometimes referred to as tracking. Tracking can enhance the performance of a communications system.
One synchronization method estimates the symbol timing and the carrier frequency offset of OFDM symbols using redundancy in a cyclic prefix of the OFDM symbols. The method includes a joint Maximum Likelihood (ML) estimator of the symbol timing and the carrier frequency offset for an additive white Gaussian noise (AWGN) channel. The joint ML estimator employs a plurality of samples in an observation window. Performance of the joint ML estimator can depend on channel conditions. When the joint ML estimator is applied to a communication system with changing channel conditions, its performance can be less than desirable. Also, the joint ML estimator assumes a signal-to-noise ratio (SNR). In practice however, the SNR is not readily available.
In another synchronization approach, reliability of the symbol timing estimate and the carrier frequency offset estimate is estimated. However, this approach requires that some OFDM symbols in the received signal contain training sequences that are known by the receiver. If the symbol timing and the carrier frequency offset estimates are not reliable, this approach needs to wait for an OFDM symbol that includes the training sequences.
Data received during one training sequence usually cannot be used with data from subsequent training sequences because the symbol timing and the carrier frequency offset typically change from one training sequence to another. Therefore, the data associated with a current training sequence is discarded if it is determined that the reliability of the training sequence was less than desirable.
Synchronization delay in the second approach can also become undesirably long if the training sequences are received when the channel is in a fading condition. Finally, a receiver using the second approach cannot synchronize if the transmitter does not insert the training sequences into at least some of the OFDM symbols.